One of the future alternatives to current fossil-based transportation fuels has been centered on hydrogen gas (H2). Currently, H2 is the primary energy source of today's space exploration projects (e.g., as rocket propellant). It is also used in fuel cells that power a variety of machinery including automobiles. Furthermore, hydrogen is an important industrial commodity produced and used in many industries. For example, it is used for the reduction of metal oxides (e.g. iron ore), ammonia synthesis, and production of hydrochloric acid, methanol and higher alcohols, aldehydes, hydrogenation of various petroleum, coal, oil shale and edible oils, among others. However, H2 is a colorless, odorless gas, and is also a flammable gas with a lower explosive limit of about 4% in air. Therefore reliable H2 sensors are required to detect possible leaks wherever H2 is produced, stored, or used.
To detect H2, sensors that consist of a palladium alloy Schottky diode on a silicon substrate are known. These sensors are based on metal-oxide-semiconductor (MOS) technology that is used in the semiconductor industry. The gas sensing MOS structures are composed of a hydrogen-sensitive metal (palladium or its alloy) deposited on an oxide adherent to a semiconductor. This hydrogen sensor has been commercialized and exploited in detecting H2 leaks during pre-launches of space vehicles. Other research groups have also used palladium or the like as a sensing element for detecting H2. A hydrogen sensor containing an array of micromachined cantilever beams coated with palladium/nickel has also been reported. Semiconductors (e.g. gallium nitride) with wide band-gap have also been used to make MOS diodes for H2 detection. One of the concerns for all of these types of sensors using palladium or the like is the requirement of a high operating temperature (greater than 200° C.) and further elevated temperatures (greater than 500° C.) to reactivate the sensing element, bringing about lengthy analysis. Another issue is sensitivity of the sensing element to other compounds commonly found in the atmosphere, including water vapor, various hydrocarbons and various reducing gases such as carbon monoxide and hydrogen sulfide.
Although not conventionally used, chemochromic sensors for hydrogen sensing have been disclosed. For example, published U.S. Application No. 20040023595 to Liu et al. discloses a fast response, high sensitivity structure for optical detection of low concentrations of hydrogen gas, comprising a substrate, a water-doped WO3 layer coated on the substrate; and a palladium layer coated on the water-doped WO3 layer. In related work, published U.S. Application No. 20040037740 to Liu et al. discloses a sensor structure for chemochromic optical detection of hydrogen gas comprising; a glass substrate a vanadium oxide layer coated on the glass substrate; and a palladium layer coated on the vanadium oxide layer. The hydrogen sensors disclosed by Liu et al. lack field stability. Moreover, such sensors have a tendency to crack and peel, and can be washed off by precipitation and/or condensation.
U.S. Pat. No. 5,849,073 to Sakamoto discloses a pigment for sensing gas leakage which can be produced by adding at least one of the salts of platinum group metals to a slurry of particulate substrate, neutralizing the resultant mixture to deposit at least one of oxides, hydroxides and hydrated oxides of platinum group metals on the surfaces of the particulate substrate, and if necessary, further adding to said slurry at least one of compounds of aluminum, silicon, titanium, zinc, zirconium, tin, antimony and cerium, neutralizing the resultant mixture to deposit at least one of compounds such as oxides, hydroxides and hydrated oxides of aluminum, silicon, titanium, zinc, zirconium, tin, antimony and cerium, on the particles. The compositions disclosed are typically quite impervious to gas penetration. Sakamoto requires very thin coatings (typically 2 mils) with relatively high concentrations of active chemochromic compounds. In addition, compositions disclosed by Sakamoto do not show selectivity to hydrogen. Thus, there remains a need for an improved, reliable and durable chemochromic hydrogen sensor for a variety of applications, including space, transportation, oil refineries, ammonia and hydrogen plants.